Amplifiers used in instrumentation circuits often require output signals responsive to differential inputs; that is the difference between inputs supplied to positive and negative input terminals of the amplifier.
Amplifiers have been designed for such purposes which provide the desired characteristics; i.e. high input impedance, high gain, excellent signal to noise ratios and high common mode rejection ratios. However, when D.C. offsets are used in the inputs of such instrumentation amplifiers, the desired characteristics are degraded drastically. Examples of differential amplifiers used with such D.C. offsets are those used in conjunction with electrocardiograph (ECG) systems.
The D.C. offsets make it extremely difficult to maintain high common mode rejection ratios. As is well known when the same input signal is applied to each of the input terminals of a differential amplifier then there should be no output voltage. In practical terms a common mode output voltage is obtained. The common mode rejection ratio is a measure of how well a differential amplifier rejects equal signals which appear simultaneously and in phase at both input terminals (the common mode signal). Thus, the common mode rejection ratio is the ratio of the common mode input voltage to the output rejection. The extent to which a differential amplifier does not provide an output voltage when the same signal is applied to both inputs is measured by the common mode rejection ratio.
It is highly desirable to have high common mode rejection ratios in instrument amplifiers such as those used in ECG systems. Nonetheless it remains a problem to those skilled in the art to provide differential amplifiers with DC offsets that do have high common mode rejection ratios.